Compositions, systems, and methods for planarizing or polishing the surface of a substrate, especially for chemical-mechanical polishing (CMP), are well known in the art. Polishing compositions (also known as polishing slurries) typically contain an abrasive material in an aqueous solution and are applied to a surface by contacting the surface with a polishing pad saturated with the polishing composition. When used for polishing substrates comprising metals, the polishing compositions often comprise an oxidizing agent. The purpose of the oxidizing agent is to convert the surface of the metals into a softer, more readily abradable material than the metal itself. Thus, polishing compositions comprising oxidizing agents in conjunction with abrasives generally require less aggressive mechanical abrasion of the substrate, which reduces mechanical damage to the substrate caused by the abrading process. Additionally, the presence of the oxidizing agent frequently increases removal rates for the metals and increases throughput in a production setting.
Development of the next generation of semiconductor devices has emphasized the use of metals with lower resistivity values, such as copper, than previous generation metals such as aluminum in order to reduce capacitance between conductive layers on the devices and to increase the frequency at which a circuit can operate. One way to fabricate planar copper circuit traces on a silicon dioxide substrate is referred to as the damascene process. In accordance with this process, the silicon dioxide dielectric surface is patterned by a conventional dry etch process to form holes and trenches for vertical and horizontal interconnects. The patterned surface is coated with an adhesion-promoting layer such as tantalum or titanium and/or a diffusion barrier layer such as tantalum nitride or titanium nitride. The adhesion-promoting layer and/or the diffusion barrier layer are then over-coated with a copper layer. Chemical-mechanical polishing is employed to reduce the thickness of the copper over-layer, as well as the thickness of any adhesion-promoting layer and/or diffusion barrier layer, until a planar surface that exposes elevated portions of the silicon dioxide surface is obtained. The vias and trenches remain filled with electrically conductive copper forming the circuit interconnects.
The polishing of a substrate containing both tantalum and copper layers typically requires the addition of a traditional copper inhibitor, such as benezotriazole (BTA) or methyl-benezotriazole (m-BTA), to the polishing slurry in order to limit the removal rate of the copper layer. The polishing of tantalum layers typically requires an oxidizing agent to achieve useful removal rates, such as a peroxide (e.g., hydrogen peroxide) or potassium iodate. Tantalum layers are typically polished at a high pH. However, the rate of copper removal remains high even for slurries having a low pH and which contain an oxidizer such as hydrogen peroxide or potassium iodate. Further, peroxide is a strong oxidizing agent that can react with other components of polishing compositions, which limits the stability of the polishing compositions and thus their useful pot-life. For example, hydrogen peroxide degrades BTA, in addition to contributing to eroding the copper lines on the substrate surface by chemical etching.
Thus, there remains a need for alternative polishing systems and methods of polishing for substrates comprising tantalum and copper.